Gas turbine engines are known to include a compressor section for supplying a flow of compressed combustion air, a combustor section for burning a fuel in the compressed combustion air, and a turbine section for extracting thermal energy from the combustion air and converting that energy into mechanical energy in the form of a shaft rotation. Many parts of the combustor section and turbine section are exposed directly to the hot combustion gasses, for example, the combustor, the transition duct between the combustor and the turbine section, and the turbine stationary vanes, rotating blades and surrounding ring segments.
It is also known that increasing the firing temperature of the combustion gas may increase the power and efficiency of a combustion turbine. Modern, high efficiency combustion turbines have firing temperatures in excess of 1,600° C., which is well in excess of the safe operating temperature of the metallic structural materials used to fabricate the hot gas flow path components. Accordingly, insulation materials such as ceramic thermal barrier coatings (TBCs) have been developed for protecting temperature-limited components. While TBCs are generally effective in affording protection for the present generation of combustion turbine machines, they may be limited in their ability to protect underlying metal components as the required firing temperatures for next-generation turbines continue to rise.
Ceramic matrix composite (CMC) materials offer the capability for higher operating temperatures than do metal alloy materials due to the inherent nature of ceramic materials. This capability may be translated into a reduced cooling requirement that, in turn, may result in higher power, greater efficiency, and/or reduced emissions from the machine. However, the required cross-section for some applications may not appropriately accommodate the various operational loads that may be encountered in such applications, such as the thermal, mechanical, and pressure loads. For example, due to the low coefficient of thermal conductivity of CMC materials and the relatively thick cross-section necessary for many applications, backside closed-loop cooling may be somewhat ineffective as a cooling technique for protecting these materials in combustion turbine applications. In addition, such cooling techniques, if applied to thick-walled, low conductivity structures, could result in unacceptably high thermal gradients and consequent stresses.
It is well known that CMC airfoils are subject to bending loads due to external aerodynamic forces. Techniques for increasing resistance to such bending forces have been described in patents, such as U.S. Pat. No. 6,514,046, and may be particularly useful for airfoils having a relatively high aspect ratio (e.g., radial length to width). However, such techniques may not provide resistance to internally applied pressures.
High temperature insulation for ceramic matrix composites has been described in U.S. Pat. No. 6,197,424, which issued on Mar. 6, 2001, and is commonly assigned with the present invention. That patent describes an oxide-based insulation system for a ceramic matrix composite substrate that is dimensionally and chemically stable at a temperature of approximately 1600° C. That patent exemplarily describes a stationary vane for a gas turbine engine formed from such an insulated CMC material. A similar gas turbine vane 10 is illustrated in FIG. 1 as including an inner wall 12. Backside cooling of the inner wall 12 may be achieved by convection cooling, e.g. via direct impingement through supply baffles (not shown) situated in relatively large interior chambers 18 using air directed from the compressor section of the engine.
If baffles or other means are used to direct a flow of cooling fluid throughout the airfoil member for backside cooling and/or film cooling, the cooling fluid is typically maintained at a pressure that is in excess of the pressure of the combustion gasses on the outside of the airfoil so that any failure of the pressure boundary will not result in the leakage of the hot combustion gas into the vane. Also, as stated above, the interior chambers 18 may be used with appropriate baffling to create impingement of the cooling fluid onto the backside of the surface to be cooled. Thus, such interior chambers enable an internal pressure force that can result in the undesirable ballooning of the airfoil structure due to the internal pressure of the cooling fluid applied to the relatively large surface area of the interior chambers 18. For example, CMC vanes with hollow cores may be susceptible to bending loads associated with such internal pressures due to their anisotropic strength behavior.
For a solid core CMC airfoil, the resistance to internal pressure depends to a large extent on establishing and maintaining a reliable bond joint between the CMC and the core material. In practice, this may be somewhat difficult to achieve with smooth surfaces and manufacturing constraints imposed by the co-processing of these materials.
For laminate airfoil constructions, the through-thickness direction has strength of approximately 5% of the strength for the in plane or fiber-direction. Stresses along the relatively weaker direction should be avoided. It is known that the internal pressure causes high interlaminar tensile stresses in a hollow airfoil, especially concentrated in the trailing edge (TE) inner radius region, but also present in the leading edge (LE) region.
This issue is accentuated in large airfoils having a relatively long chord length, such as those used in large land-based gas turbines. The longer internal chamber size results in increased bending moments and stresses for a given internal pressure differential.
One known technique for dealing with these stresses is the construction of internal spars 14 disposed between the lower and upper surfaces of the inner wall 12. The internal spars may extend, either continuously or in segmented fashion, from one side of the airfoil to an opposite side of the airfoil. However, construction of such spars for CMC vanes involves some drawbacks, such as due to manufacturing constraints, and thermal stress that develops due to differential thermal growth at the hot airfoil skin and the relatively cold spars 14, as well as thermal gradient present at the root of the spar. The resulting thermal stress may cause cracks to develop at the intersection of the spars and the inner wall leading to failure of the turbine foil.
Therefore, improvements for reducing bending stresses resulting from internal pressurization of an airfoil are desirable.